(1) Field of the Invention
The present invention relates to a biaxially oriented heat-resistant bottle, particularly to a bottle having the heat-resistant bottom portion which can be prepared by stretching and blow molding a thermoplastic resin such as polyethylene terephthalate.
(2) Description of the Prior Art
A self-supporting bottle which can be prepared by axially stretching and air blowing a preform of polyethylene terephthalate is more excellent in durability as compared with a bottle made by blowing molding, but if filled with heated contents, the stretched and air blown bottle must be subjected to a heat treatment so that it may not be contracted and deformed by heating at the filling step. This heat treatment is extremely effective to a sufficiently diaxially oriented body portion and bottom peripheral wall of the bottle, but it is impossible to cause even the bottom wall to have a thermal stability. Thus, most of the bottles are short of the thermal stability to induce the heat deformation on their bottom walls.
The heat resistance of this bottom wall is considered to be improved by attaining a sufficient biaxial orientation also on the bottom wall, but it is difficult to biaxially orient sufficiently even the central portion of the bottle which is restricted in draw ratio from the viewpoint of its bottom structure. Even if the biaxial orientation is attained, the bottom wall, which is recessed inwardly for the betterment of its self-supporting character, will be thin and will expand outwardly by an applied load when filled with contents, with the result that the self-supporting ability of the bottle will be lost.
The self-supporting bottom structure which is generally called a champagne bottom is formed into the shape of a dome, as illustratively shown in FIG. 4, by recessing, toward the interior of a bottle body 11, a bottom wall 13 continuous with a peripheral wall 12 which is sufficiently biaxially oriented together with the bottle 11. Supporting the bottle 1 itself is accomplished by an annular peripheral edge 14 formed between the peripheral wall 12 and the bottom wall 13.
In the case of such a bottom structure, when the bottle 1 is molded in a blow mold 15 by axially stretching and air blowing a parison 16, a bottom portion 16a of the parison 16 will be brought into contact with a mold bottom 17 earlier than any other portion thereof and will be cooled. Further, the draw ratio of the parison bottom portion will be small due to the structure of the mold bottom portion 17. Therefore, the bottom wall 13 having a dome shape will be thick-wall in its central portion 13a as shown in FIG. 4, and the biaxial orientation will be accomplished only in the sufficiently stretched thin annular peripheral edge 14 and a peripheral portion 13b adjacent thereto.
In the bottom portion having the above-mentioned structure, a portion 13c which is insufficient in the biaxial orientation and is relatively thin-wall is apt to be deformed by heating. The central portion 13a also tends to be thermally deformed, but since having a thick wall, it can withstand the thermal influence to some extent and can prevent the bottom wall 13 from being badly deformed.
However, the portion 13c where the orientation is poor is thinner as compared with the central portion 13a, therefore when the bottle is filled with contents heated up to a temperature of 75.degree. C. or more, some deformation will be brought about thereon. The portion 13c will often be bulged out of the annular edge 14, when loaded.
Such phenomena will impair the self-supporting ability of the bottle, and even if such an impairment is not reached, it is sure that the shape of the bottom will be ugly and its commercial value will be lowered.
As means for providing the portion 13a, which includes the poor orientation and will thus easily be thermally affected, with a heat resistance by achieving the sufficient biaxial orientation, there if formed a hollow rib 18 which is shown by a chain line at the intermediate portion of the bottom wall 13.
In most cases, however, such a rib 18 is straight or is somewhat transversely bulged or curved out toward the wall surface of the dome-shape bottom wall 13 owing to the restriction on the blow molding. In short, the rib 18 is not formed so as to noticeably project from the wall surface.
As a result of the formation of such a rib 18 which is slightly bulged or curved out, a bottom area necessary for the biaxial orientation increases only a little. Therefore, the central portion 13a in contact with the rib 18 is insufficiently stretched, thereby remaining thick, though the peripheral portion 13b in contact therewith is sufficiently done. For this reason, the functional effect of the rib 18 is to improve a heat resistance only for the peripheral portion 13b of the bottom wall 13, and the above-mentioned portion 13c which is susceptible to a heat influence cannot be provided with the heat resistance. Accordingly, even if the rib 18 is formed, the bottle will be deformed, when packed with heated contents.